Pascal

1. Pascal Prof. Steven A. Demurjian Computer Science & Engineering Department The University of Connecticut 371 Fairfield Way, Box U-255 Storrs, CT 06269-3255 Steven.Demurjian@uconn.edu http://www.engr.uconn.edu/~steve (860) 486–4818 (Office) (860) 486-3719 (CSE Office)

2. Getting Started With Pascal Programming http://pages.cpsc.ucalgary.ca/~tamj/2002/231P/notes/starting_programming/starting_pascal_programming.ppt http://pages.cpsc.ucalgary.ca/~tamj/2002/231P/notes/pascal/

3. Basic Structure Of Pascal Programs Program name.p (Pascal source code) Part I: Header Program documentation program name (input, output); Part II: Declarations const : Part III: Statements begin : end.

4. Details Of The Parts Of A Pascal Program • Headers • Program documentation • Version number, date of last modification, what does the program do etc. • Comments for the reader of the program (and not the computer) (* Marks the beginning of the documentation *) Marks the end of the documentation • Program heading • Name of program, input and/or output operations performed by the program • Example (* * Tax-It v1.0: This program will electronically calculate your tax return. *) program taxIt (input, output);

5. The Smallest Pascal Program • (* • * Smallest.p: • * The smallest Pascal program that will still compile written by James Tam • * v1.0. • *) • program smallest; • begin • end.

6. Variables • Set aside a location in memory • Used to store information (temporary) • Types: • integer – whole numbers • real – whole numbers and fractions • Can't start or end with a decimal • char – alphabetic, numeric and miscellaneous symbols • boolean – true or false values • Usage: • Declaration • Accessing or assigning values to the variables

7. Declaring Variables (2) • Occurs in the variable declaration ("var") section • i.e., • var • name of first variable, name of second variable…: type of variables; • e.g., • var • height, weight: real; • age: integer;

8. Not allowed! Accessing And Assigning Values To Variables (2) • Assignment • Performed via the assignment operator := • Usage: • Destination := Source;1 • Example: • x := 5; • x:= y; • interest := principle * rate; • character := 'a'; • Avoid assigning mixed types e.g., var num1: integer; num2: real; begin num1 = 12; num2 = 12.5; num2 := num1; 1 The source can be any expression (constant, variable or formula) num1 := num2;

9. Named Constants (2) • Examples: • const • TAXRATE = 0.25; • SAMPLESIZE = 1000; • YES = True; • NO = False;

10. Output • Displaying information onscreen • Done via the write and writeln statements • Syntax (either write or writeln): • write ('text message'); • or • writeln('text message'); • write(name of variable or constant); • or • writeln (name of variable or constant); • write('message', name of variable, 'message'…); • or • writeln('message', name of variable, 'message'…);

11. Output (2) • Examples: • var • num : integer; • begin • num := 10; • writeln('line1'); • write('line2A'); • writeln('line2B'); • writeln(num); • writeln('num=',num);

12. Formatting Output • Automatic formatting of output • Field width: The computer will insert enough spaces to ensure that the information can be displayed. • Decimal places: For real numbers the data will be displayed in exponential form. • Manually formatting of output: • Syntax: • write or writeln (data: Field width for data: Number decimal places for data);

13. Formatting Output (2) • Examples • var • num : real; • begin • num := 12.34; • writeln(num); • writeln(num:5:2);

14. Formatting Output (3) • If the field width doesn’t match the actual size of the field • Field width too small – extra spaces will be added for numerical variables but not for other types of data. • Examples: num := 123456; writeln(num:3); writeln('123456':3); • Field width too large – the data will be right justified (extra spaces will be put in front of the data). • Examples: num := 123; writeln(num:6); Writeln('123':6);

15. Formatting Output (4) • If the number of decimal places doesn’t match the actual number of decimal places. • Set number of decimal places less than the actual number of decimal places – number will be rounded up. • Example: num1 := 123.4567 writeln (num1:6:2); • Set number of decimal places greater than the actual number of decimal places – number will be padded with zeros. • Example: num1 := 123.4567; writeln(num1:6:6);

16. Formatting Output: A Larger Example • program out1; • var • num1 : integer; • num2 : real; • begin • num1 := 123; • num2 := 123.456; • writeln('Auto formatted by Pascal', num1, num2); • writeln('Manual format':13, num1:3, num2:7:3); • writeln('Manual not enough':13, num1:2, num2:6:3); • writeln('Manual too much':16, num1:4, num2:8:4); • end.

17. Input • The computer program getting information from the user • Done via the read and readln statements • Syntax: • (single input) • read (name of variable); • or • readln (name of variable); • (multiple inputs) • read (nv1, nv2…); • or • readln (nv2, nv3…);

18. Input (2) • Examples: • var • num1, num2 : integer • begin • read (num1); • read (num2); • read (num1, num2);

19. Input: Read Vs. Readln • Both: • Reads each value inputted and matches it to the corresponding variable. • Read • If the user inputs additional values they will remain • Readln • Any additional values inputted will be discarded

20. Input: Read Vs. Readln (An example) • write( some integers making sure to separate each one with a space '); • write('or a new line: '); • read (num1, num2); • write('Input some integers making sure to separate each one with a space '); • write('or a newline: '); • read(num3, num4);

21. Input: Read Vs. Readln (An example (2)) • write('Input some integers making sure to separate each one with a space '); • write('or a newline: '); • readln (num1, num2); • write('Input some integers making sure to separate each one with a space '); • write('or a newline: '); • readln(num3, num4);

22. Extra Uses Of Readln • To filter out extraneous input • As an input prompt • e.g., • writeln('To continue press return'); • readln;

23. Remaining Topics • Arrays • Conditionals • Looping • Files • Multi-Dimensional Arrays • Files • Functions/Procedures • Pointers and Lists • \

24. Arrays • const • CLASSSIZE = 5; • var • classGrades : array [1..CLASSSIZE] of real; • i : integer; • total, average : real; • begin • total := 0;

25. for i := 1 to CLASSSIZE do • begin • write('Enter grade for student no. ', i, ': '); • readln (classGrades[i]); • total := total + classGrades[i]; • end; • average := total / CLASSSIZE; • writeln; • writeln('The average grade is ', average:6:2, '%'); • for i := 1 to CLASSSIZE do • writeln('Grade for student no. ', i, ' is ', classGrades[i]:6:2, '%');

26. No semi-colon (indicates end of decision-making!) CONDITIONALS: If-Then-Else (Simple Body) • Body of if-then-else consists of a single statement • Format: • if (Boolean Expression) then • s1 • else • s2; • s3;

27. If-Then-Else (Simple Body(2)) • if (x = 1) then • writeln('body of if') • else • writeln('body of else'); • writeln('after if-then-else');

28. No semi-colon (marks end of decision-making!) If-Then-Else (Compound Body) • Body of if-then-else consists of multiple statements • Format: • if (Boolean Expression) then • begin • s1; • : • sn; • end • else • begin • sn+1; • : • sn + m; • end; • Sn + m + 1;

29. If-Then (Compound Body(2)) • if (x = 1) then • begin • writeln('Body of if 1'); • writeln('Body of if 2'); • end • else • begin • writeln('Body of else 1'); • writeln('Body of else 2'); • end; • writeln('after if-then-else');

30. Case Statements • An alternative to the if, else-if (only one condition is true) • Format (integer): • Case (expression) of • i1: • body; • i2: • body; • : • in: • body; • otherwise: • body; • Expression (variable, constant, arithmetic) must evaluate to an integer

31. Case Statements: Integer Example • case (gpa) of • 4: • begin • writeln(‘You got an A’); • end; (* GPA of 4 *) • 3: • begin • writeln(‘You got a ‘B’); • end; (* GPA of 3 *) • 2: • begin • writeln(‘You got a C’); • end; (* GPA of 2 *) • 1: • begin • writeln(‘You got a D’); • end; (* GPA of 1 *) • 0: • begin • writeln(‘You got an F’); • end; (* GPA of 0 *) • end; (* case *)

32. Case Statements: Characters • Format (char): • Case (expression) of • ‘c1’: • body; • ‘c2’: • body; • : • ‘cn’: • body; • otherwise • body; • Expression (variable, constant, arithmetic) must evaluate to a character

33. Case Statements: Character Example • case (letter) of • ‘A’: • begin • writeln(‘GPA = 4’); • end; (* GPA of 4 *) • ‘B’: • begin • writeln(‘GPA = 3’); • end; (* GPA of 3 *) • ‘C’: • begin • writeln(‘GPA = 2’); • end; (* GPA of 2 *) • ‘D’: • begin • writeln(‘GPA = 1’); • end; (* GPA of 1 *) • ‘F’: • begin • writeln(‘GPA = 0’); • end; (* GPA of 0 *) • end; (* case *)

34. Initialize control Test condition Execute body Update control Pre-Test Loop: While-Do • Can be used if the number of times that the loop executes is not known in advance. • Syntax: • while (Boolean expression) do • body • i: = 1; • while (i <= 10) do • begin • writeln('i = ', i); • i := i + 1; • end; (* while *)

35. Initialize control Update control Test condition Execute body First For Loop Example • var • i, total : integer; • begin • total := 0; • for i := 1 to 10 do • begin • total := total + i; • writeln('i = ':4, i:2, 'total = ':13, total:2); • end; (* for *) • end.

36. Second For Loop Example • var • i, total : integer; • begin • total := 0; • for i := 5 downto 1 do • begin • total := total + i; • writeln('i = ':4, i:2, 'total = ':13, total:2); • end; (* for *) • end.

37. Post Test Loops: Repeat-Until • Used instead of a while-do loop if you need the loop to execute at least once. • Syntax: • repeat • body • until (Boolean expression);

38. Test condition Execute body Initialize control Update control Repeat-Until: An Example (2) • repeat • begin • answer := trunc(Random * MAX); • write('Enter your guess: '); • readln(guess); • if (guess = answer) then • writeln('You guessed correctly!') • else • writeln('You guessed incorrectly'); • writeln('Play again?'); • writeln('Enter "Y" or "y" to play again'); • writeln('Enter "N" or "n" otherwise'); • write('Choice: '); • readln(choice); • end; • until (choice = 'N') OR (choice = 'n');

39. Infinite Loops • Loops that never end (the stopping condition is never met). • Infinite loops can be caused by logical errors: • The loop control is never updated (Example 1). • The updating of the loop control never brings it closer to the stopping condition (Example 2). • i := 1; • while (i < 10) do • writeln ('i = ', i); To stop a program with an infinite loop in Unix simultaneously press the <ctrl> and the <c> keys

40. Nested Loops • One loop executes inside of another loop(s). • Example structure: • Outer loop (runs n times) • Inner loop (runs m times) • Body of inner loop (runs n x m times) • for i := 1 to 2 do • for j := 1 to 3 do • writeln('i = ':9, i, 'j = ':9, j); • writeln('All done!');

41. FILES: Indicating What File You Are Reading From • Syntax: • program name (name of input file1); • Example: • program grades (output, letterGrades); • A variable that is associated with a file on disk • Syntax: • name of file2 : text; • Example: • letterGrades : text; 1 The name of input file must correspond to an actual file in the same directory that the executable program resides 2 The name of file variable must correspond to an actual file in the same directory that the executable program resides

42. Opening Files • Purpose: • Prepares the file for reading (positions the file pointer) • Syntax: • reset (name of input file3); • Example: • reset(letterGrades); 3 The name of file being opened must correspond to an actual file in the same directory that the executable program resides

43. Reading Information From Files • Performed with read or readln • Syntax: • read (name of input file4, variable(s)); • readln (name of input file4, variable(s)); • Example: • readln(letterGrades, letter); 4 The name of file being read from must correspond to an actual file in the same directory that the executable program resides

44. Reading Information From Files (2) • Typically reading is done within the body of a loop • Syntax: • while NOT EOF (name of input file5) do • begin • read (name of input file5, variable(s)); • readln (name of input file5, variable(s)); • end; (* Done reading from input file *) • Example: • while NOT EOF (letterGrades) do • begin • readln(letterGrades, letter); • writeln(letter); • end; (* Loop to read letter grades file *) 5 The name of the input file must correspond to an actual file in the same directory that the executable program resides

45. Alternative Approach To Reading Files • Employ a sentinel in the file • Keep reading from the file until the sentinel value is encountered • Example: • var • inputFile : text; • num : integer; • : : • readln (inputFile, num); • while NOT (num = -1) do • begin • writeln(num); • readln(inputFile, num); • end; (* Done reading input file *)

46. Reading From Files: Putting It All Together • program grades (output, letterGrades); • var • letterGrades : text; • letter : char; • begin • reset(letterGrades); • writeln('Opening file "letterGrades" for reading.'); • while NOT EOF (letterGrades) do • begin • readln(letterGrades, letter); • writeln(letter); • end; (* Loop to read letter grades file *) • close(letterGrades); • writeln('Completed reading of file "letterGrades"'); • end. (* End of program *)

47. Opening The File • Two methods: • Rewriting – erases the old contents of the file (rewrites over what was already there). • Appending – retain the old contents of the file (appends the new information at the end). • Syntax (rewriting / appending): • rewrite (name of file); append (name of file); • Example (rewriting / appending): • rewrite(gradePoints); append(gradePoints);

48. Writing To A File: Putting It All Together • program grades (output, letterGrades, gradePoints); • var • letterGrades : text; • gradePoints : text; • letter : char; • gpa : integer; • begin • reset(letterGrades); • rewrite(gradePoints); • writeln('Opening file "letterGrades" for reading.'); • writeln('Opening file "gradePoints" for writing.'); • while NOT EOF (letterGrades) do • begin

49. Writing To A File: Putting It All Together (2) • readln(letterGrades, letter); • case (letter) of • 'A' : gpa := 4; • 'B' : gpa := 3; 'C' : gpa := 2; • 'D' : gpa := 1; • 'F' : gpa := 0; • otherwise gpa := -1; • end; (* case *) • writeln(gradePoints, gpa); • end; (* Loop to read letter grades file *) • writeln('Finished reading and writing to files.'); • close(letterGrades); • close(gradePoints); • end.

50. Details Of Write And Writeln For Files: Intuitive View • Program statement Effect on file • rewrite (data); (Open file "data" and position • ^ file pointer at start) • write (data, 'x'); x • ^ • write(data, 'y'); xy • ^ • write(data, 'z'); xyz • ^ • writeln(data); xyz • _ • ^ • write(data,'a'); xyz • a • ^